The descriptions in this section merely provide background information of embodiments of the present disclosure and are not intended to specify prior arts of the present disclosure.
Emergences of next generation mobile communication systems such as 5th generation (5G) mobile networks increase the necessity of a wireless transmission technology capable of transmitting high speed data of one Gigabytes per second (Gbps) or faster. Meanwhile, the radio frequency of a millimeter-wave band, in which it is easy to secure a bandwidth of several hundred MHz or more, is getting more attention than ever. At the time this International application is being filed, the 3rd Generation Partnership Project (3GPP) has begun discussing the standardization of the 5G networks, and the other companies and organizations are actively discussing standard technologies and developing element technologies of the 5G system.
Here, the millimeter-wave refers to an electromagnetic wave having a frequency of 30 GHz or higher, for example, 30-300 GHz. In particular, frequencies of 28 GHz, 38 GHz, 60 GHz, and 70 GHz are currently being considered as frequencies for use in the 5G network.
A signal in the frequencies of the millimeter-wave band experiences a higher transmission loss in the air and undergoes less diffraction than the frequencies in a conventional 4G frequency band, Thus, a beamforming technique for concentrating radio waves in a desired direction using a plurality of antennas will generally be utilized for the wireless transmission.
The beamforming refers to a signal processing technique for directional signal transmission or reception in such a way that the energy radiated from or received by an antenna is concentrated in a particular direction in space. The beamforming allows to receive a stronger signal from a desired direction or to transmit a stronger signal in a desired direction while reducing a signal transmitted to or received from an undesired direction.
In the conventional 4G frequency band, digital beamforming which adjusts both amplitude and phase of a signal in a digital baseband is used generally. However, in a higher frequency band including the millimeter-wave band, an analog beamforming is expected to be used due to the complexity of a radio frequency (RF) stage and an analog-to-digital/digital-to-analog (AD/DC) converter and power consumption issues. In particular, de facto standards of IEEE 802.15.3c and 802.11ad for a wireless personal area network (PAN) and a local area network (LAN), respectively, using millimeter-wave 60 GHz correspond to examples which adopts the analog beamforming.
A hybrid beamforming which combines features of the digital beamforming and the analog beamforming may be used in a base station having relatively less complexity than a terminal. The hybrid beamforming utilizes the flexibility and the multilayer transmission capability of the digital beamforming and the simplicity of the analog beamforming.
A massive multiple-input multiple-output (MIMO) may be implemented by cost-effectively increasing the number of antennas through the hybrid beamforming. Further, a plurality of beamforming signals may be generated simultaneously in case of the hybrid beamforming. Accordingly, the hybrid beamforming allows the system to transmit the beamforming signal to a plurality of users using one frequency-time resource. Also, the hybrid beamforming may increase a signal-to-noise ratio (SNR) and enhance a frequency efficiency.
However, in this case, an interference problem may occur between beamforming signals transmitted to a plurality of users by use of the same frequency-time resource. Particularly, when the analog beamforming is utilized in the hybrid beamforming architecture, a sidelobe being transmitted in a direction other than the beamforming direction may become a problem.
Of course, it is possible to reduce the interference between the users by using MIMO digital processing. However, in this method, it is difficult to implement a rich channel appropriately, and there arises a problem such as a limitation of calculation. Accordingly, it is necessary to control the inter-user interference at the RF stage and the analog beamforming stage.